1. Introduction
This invention relates to removal of metal ions from organic solvents. The invention is especially useful for the removal of metal ions from organic solvents and solutions of photoresist components.
2. Description of the Prior Art
It is known in the art that organic solvents are often contaminated with metal ions. This contamination is typically due to raw materials used to manufacture the solvent, the reactants, and the reaction vessels in which the solvents are prepared.
The level of metal ion contamination in a solvent may range up to about 500 parts per million parts of solvent (ppm) or more dependent upon the solvent and the manner in which the solvent is prepared. For many commercial applications, this level of metal ion contamination is acceptable. However, for other commercial applications, such as use of organic solvents in the manufacture of pharmaceuticals, radio pharmaceuticals, processing chemicals such as functionalized silanes for optical fibers, adhesion promoters, acid sensitive hydraulic fluids, dielectric fluids, reference standards and in the manufacture of chemicals for microlithography, this level of contamination is unacceptable. Using microlithography and the manufacture of integrated circuits for purposes of illustration, sodium and potassium ions are particularly problematic. According to Elliot, Integrated Circuit Fabrication Technology, McGraw-Hill Book Company, New York, pp. 312-313, 1982, most positive photoresists have a total metal content of less than 1 ppm. Moreover, it is the current goal of photoresist manufacturers to reduce the metal ion content of such photoresists to levels not exceeding 100 parts per billion (ppb) and preferably, not exceeding 10 ppb.
It is known in the art to remove metal ion contaminants from photoresist components using highly acidic cation exchange materials. The removal of such contaminants using cation exchange materials is disclosed in U.S. Pat. No. 5,234,789 and Japanese applications Nos. 5148309, 91339728, 5148306 and 4065415, each incorporated herein by reference for their teaching of the use of cation exchange resins for purifying photoresist components and for the disclosure of suitable cation exchange materials for that purpose.
Ethyl lactate is a known and commonly used solvent for photoresists. It is known from U.S. Pat. No. 5,234,789 that the use of cation exchange resins to purify ethyl lactate solvents at elevated temperatures results in transesterification and saponification reactions with the formation of a lactide and other by-products. In accordance with said patent, the presence of a polylactide in a photoresist composition is advantageous as the lactide is said to function as a lithographic speed enhancer. However, it has now been discovered that the advantage of speed enhancement is outweighed by disadvantages resulting from the presence of by-products such as degradation of the thermal properties of a photoresist coating containing the lactide and difficulty in controlling the photospeed of the photoresist during manufacture.
In addition to the formation of lactides in ethyl lactate, it is also known that undesirable by products are formed when most acid labile materials are treated with a cation exchange resin. Therefore, it is desirable to find a method by which metal ion contaminants can be removed from acid labile materials without the formation of undesirable by-products.